1. Field of Invention
This invention relates generally to frequency correction, and more particularly to a method and apparatus for performing automatic frequency offset detection and correction in broadband wireless access systems.
2. Discussion of Background
Point to multipoint fixed broadband wireless access systems over MMDS networks are known in broadcast situations. These networks operate over licensed bands including the MMDS band (2,150 to 2,162 MHz), the WCS band (2,305 to 2,360 MHz) and the ITFS/MMDS bands (2,500 to 2,686 MHz).
A known cable based broadband access system, which operates at a range of between 50 MHz and 864 MHz, but not in the MMDS, WCS, or ITFS/MMDS bands, is the data over cable specification system, which is specified in the data over cable system interface specifications (DOCSIS). An overview of a cable based DOCSIS system is depicted in FIG. 1. A CMTS 10 communicates with a wide area network 20, such as the Internet. The CMTS 10 can transmit signals from the wide area network 20 along a cable network 30 through cable modems 40 to CPE 50 (Customer Premise Equipment—intended throughout this document to include a computer and/or all of the equipment at the customer site, such as a LAN—Local Area Network). CPE 50 messages can be transmitted to the wide area network 20 through the cable modem 40 along the cable network 30 to the CMTS 10.
In point to multipoint broadband access systems one central end-point, e.g. the head-end, communicates through a bi-directional link or links with multiple end-points, e.g. the nodes. The number of nodes in communication varies in time and can be none, one or two or more at any specific time.
The link(s) between the head-end and the nodes are combined in one or more channels. The signal path from the central end-point to the nodes is referred to as downstream, while the signal path from the nodes to the central end-point is referred to as upstream.
A single upstream channel can be used to deliver information from a node to the head-end, and a downstream channel from the head-end to a node or a group of nodes. If a single upstream channel is used for communication from the nodes(s) to the central point, then only one end-point can sends information on the single upstream channel at any one time.
Determining and maintaining accurate frequencies for downstream reception and upstream transmission is important for successful system operation. One way to determine and maintain accurate frequencies is to use sensitive but expensive circuit elements in the transceiver, or receiver and transmitter. The use of sensitive component will improve the frequency accuracy of the modem but will also lead to an expensive modem, making it difficult to sell the device at a reasonable price to the consumer.
In broadband access systems a very important time for a modem to be accurately detect and calibrate frequencies is when it wants to initiate communication with a cable modem termination system (CMTS) and therefore needs to register and be able to operate according to the appropriate parameters of the broadband wireless access system. It is during the registration procedure that modem needs to accurately determine the downstream and upstream frequencies in order to accurately transmit and receive during its communication period.
Therefore there exists a need for a modem that can accurately lock onto and maintain the downstream frequency in order to receive a reliable downstream data.
There also exists a need for a modem that can accurately lock onto and maintain the upstream frequency in order to transmit in the exact frequency without disturbing neighboring upstream channels.
However, it is further desired that creating a modem that can accurately lock onto and maintain both the downstream and upstream frequencies while at the same time reducing the need to use expensive components.
Another frequent problem in broadband wireless access systems occurs due to line of sight operation. The problem is very short duration interruptions in the in the physical transmission path between the wireless hub and the wireless modems is regarding very fast interruptions. The wireless modem needs to be able to make a fast reaction in order not to have data loss in its transmissions after these situations.
Therefore, it is desired that modem have the capability to compensate for short duration interruptions in the transmission path.